My 5 cents. Children are malleable. They get strongly affected by things we think are trivial and they don’t even register things that we think are traumatic. There is one certainty, though, children DO notice the way their parents react to situations and they learn from that. 5 year olds do not struggle with morality the way adults do, they simply take in information, process it, then react accordingly.
I understand that your morality is different from your STBX. (maybe that’s why he’s an ex, idk.) I do agree with young_fool that your original post does reek of disapproval and control. (e.g. the mention of overnight visits…if he didnt’ get the GF pregnant while the son was in the house, then why bring this up? Obviously he didn’t because the son ‘has no knowledge of this girlfriend’. It is immaterial to the matter at hand.)
Adjusting the Xmas visitation because you disapprove of your ex’s lifestyle and decision, is simply an attempt to control something that will have to have to come out in the wash one way or another…and soon. You cannot prevent him from learning about the unmarried, pregnant GF or his soon to be half-sibling, but you can be there to help him adjust to the situation.
You have a right to raise your child with the morals you sit fit, but so does your ex. Choosing to have a child while unmarried isn’t an absence of morals, but a different set of morals. It is his right to inform his 5 year old of a pending younger sibling. Whether this is to be done during or after Xmas is between the two of you.
Your ex will have to tell the child something soon, otherwise it’ll appear as though he hid it (lied?) from the child’s perspective. And what happens if he meets the GF and asks why she is so fat? Should the ex lie because it’ll make you feel more comfortable? Worse, what happens if you do prevent your son from learning about the GF and sibling until after the sibling is born? That’s a whole other world of potential problems right there.
Personally, I think springing both the GF AND the baby on him at the same time is a mistake. It would be overwhelming for your son to have to process both at the same time. He should’ve introduced the GF a while back so that your son didn’t get hit with the double-whammy at once. However, I don’t think preventing the child from seeing the father at Xmas is the answer.